1. Field of the Invention
The present invention relates to a coordinate input system using a tablet provided for detecting an axial coordinate. The coordinate input system using the tablet was designed for a small personal computer, and can be carried with the computer.
2. Related Art
A coordinate input system using the tablet has an advantage in that it is able to combine a liquid crystal indication screen and a resistance membrane tablet in one body.
A resistance membrane tablet of the coordinate input system has a film side resistance membrane for the X coordinate input and a glass side resistance membrane for the Y coordinate input. The film side resistance membrane consists of transparent matter. An insulating material is sandwiched between the film side resistance membrane and the glass side resistance membrane. The insulating material breaks down under pressure.
When a stylus pen for indicating a coordinate touches the film side resistance membrane, the film side resistance membrane and the glass side resistance membrane contact. As a result, a resistance division occurs on both the transparent film side resistance membrane and the glass side resistance membrane. In this condition, electrodes on each resistance membrane detect the electrical potential of each resistance membrane.
The electrical potential detected by each electrode is converted into a digital signal by an A/D converter (analog-digital converter). CPU calculates the X coordinate and the Y coordinate from this digital signal.
The known coordinate input system has four electrodes or a eight electrodes. FIG. 6 shows a structure of a resistance membrane tablet having four electrodes. FIG. 7 shows a structure of resistance membrane tablet having eight electrodes. FIG. 8 shows an equivalent circuit of the resistance membrane tablet with four electrodes. FIG. 9 shows an equivalent circuit of the resistance membrane tablet with eight electrodes.
The resistance membrane tablet with four electrodes has an input side 10, four electrodes 11 (M1, M2, M3, M4), and four lines 12 (A1, A2, A3, A4). The resistance membrane tablet with eight electrodes has an input side 10, eight electrodes 21 (M1, M2, M3, M4, N1, N2, N3, N4), and eight lines 22 (A1, A2, A3, A4, A5, A6, A7, A8). The area of the electrode part of the tablet with eight electrodes is about 2 times the area of the electrode part of the tablet with four electrodes.
Next, an operation of detecting of coordinates is explained for the resistance membrane tablet with four electrodes. When the stylus pen PE contacts a point Pf on the input side 10, the film side resistance membrane 30 (Resistance value is Rf) contacts the glass side resistance membrane 31 (Resistance value is Rg). As a result a resistance division Rf1, Rf2, Rg1 and Rg2 occurs. Contact resistance 70 (R1) and 71 (R2) occur at the connection points with the line 12 (A1, A2) and a connector 44.
By closing switches SW1 and SW2, the circuit containing the resistance division 70 (R1) and 71 (R2) and the film side resistance membrane 30 (Rf) closes. Then it supplies a voltage V (+5 v) to each electrode M1 and M2 at both ends of the film side resistance membrane 30 (Rf) through the line A1 and A2. A value of electrical potential of the electrode M1 is established to 5 volts, and a value of electrical potential of the electrode M2 is established to 0 volt (ground). On the other hand, by opening switch SW3 and SW4, the circuit containing the resistance division 72 (R3) and 73 (R4) and the glass side resistance membrane 31 (Rg) opens. Then it doesn't supply the voltage to each electrode M3 and M4, at both ends of the glass side resistance membrane 31 (Rg).
In this condition, a potential drop of the resistance division Rf1 and Rf2 occurs at the film side resistance membrane 30. Then, a value of electric potential of a point Pf of the film side resistance membrane 30 becomes Vf. Equation (1) expresses the electric potential of the point Pf of the film side resistance membrane 30. EQU Vf=(Rf2+R1)/(Rf+R1+R2).times.V (1) EQU Rf=Rf1+Rf2 (2)
Because an electric current does not spread in the glass side resistance membrane 31, a potential drop of the resistance division Rg1 and Rg2 does not occur at the glass side resistance membrane 31. Accordingly, the value of the electrical potential detected by the electrode M3 or M4 of the glass side resistance membrane 31 is the same as the electric potential of the point Pf of the transparent film side resistance membrane 30. Both the electrical potential detected by the electrode M3 and the electrical potential detected by the electrode M4 have the same value, the value of the electrical potential of the point Pf of the film side resistance membrane 30. A signal of electrical potential of the point Pf of the film side resistance membrane 30 detected by the electrode M3 or M4 is taken out from the line A3 or A4 which was connected to each electrode (M3, M4)
FIG. 2 shows a functional structure of the coordinate input system using a tablet. The lines A3 and A4 of the glass side resistance membrane 31 are connected to the A/D converter 2. The A/D converter 2 converts the electrical potential of the point Pf of the film side resistance membrane 30 which has analog quantities into a digital signal. A CPU3 calculates the X coordinate from the digital signal from the A/D converter 2.
For example, a position of the electrode M1 has a starting point of X coordinate (X=0), and a position of electrode M2 has a terminal of X coordinate (X=1024). The intervals from electrode M1 to electrode M2 are then divided into a value of 1024 dots. It is then assumed that a value of the resistance division 70 and a value of the resistance division 71 are 0 (R1=R2=0). In this case, when the stylus pen PE points at a position of the electrode M1 in order to point at a position of X=0 dots, electrical potential (Vf) of the point PE of the film side resistance membrane 30 becomes V volts. This electrical potential of the point PE of the film side resistance membrane 30 is detected by the electrode M3 or M4. An expression of electrical potential of a point PE is shown with the next expression. EQU Vf=(Rf+0)/(Rf+0+0).times.V=V (3) EQU Rf2=Rf (4)
The CPU3 calculates a value of X coordinate from the electrical potential of point PE of the film side resistance membrane 30. As a result CPU3 detects that X coordinate is X=0.
On the other hand, when the stylus pen PE points at a position of the electrode M2 in order to point at a position of X=1024 dots, electrical potential (Vf) of the point PE of the film side resistance membrane 30 becomes 0 volts. This electrical potential of the point PE of the film side resistance membrane 30 is detected by the electrode M3 or M4. In this case, the expression of electrical potential of a point PE is shown with the next expression. EQU Vf=(0+0)/(Rf+0+0).times.V=0 (5) EQU Rf2=0 (6)
The CPU3 calculates a value of the X coordinate from the electrical potential of point PE and detects that X coordinate as X=1024.
When the stylus pen PE points at an intermediate point of electrode M1 and electrode M2 in order to point at a position of X=512 dots, electrical potential (Vf) of the point PE of the film side resistance membrane 30 becomes 1/2V volts. This electrical potential of the point PE of the film side resistance membrane 30 is detected by the electrode M3 or M4. In this case, the expression of electrical potential of a point PE is shown with the next expression. EQU Vf=(1/2Rf+0)/(Rf+0+0).times.V=1/2V (7) EQU Rf2=1/2Rf (8)
The CPU3 calculates a value of X coordinate from the electrical potential of point PE and detects that X coordinate as X=512.
In case of detecting Y coordinate, by closing the switch SW3 and SW4, the circuit containing the resistance division 72 (R3) and 73 (R4) and the glass side resistance membrane 31 (Rg) closes. Then it supplies voltage V (+5v) to each electrode M3 and M4 at the ends of the glass side resistance membrane 31. By opening the switch SW1 and SW2, the circuit containing the resistance division 70 (R1) and 71 (R2), and the film side resistance membrane 30 (Rf) a opens. And a value of electrical potential of the electrode M3 is established to 5 volts, and a value of electrical potential of the electrode M4 is established to 0 volt (ground). In this condition, a resistance division Rg1 and Rg2 occurs at a point Pg of the glass side resistance membrane 31. The potential drop of the resistance division Rg1 and Rg2 occurs at the glass side resistance membrane 31, then a value of electric potential of the point Pg of the glass side resistance membrane 31 becomes Vg. An expression (9) shows an expression of the electric potential (Vg) of the point Pg of the glass side resistance membrane 31. EQU Vg=(Rg2+R3)/(Rg+R3+R4).times.V (9) EQU Rg=Rg1+Rg2 (10)
This electrical potential of the point Pg of the glass side resistance membrane 31 is detected by the electrode M1 or M2. A signal of electrical potential of the point Pg of the glass side resistance membrane 31 detected by the electrode M1 or M2 is taken out from the line A1 or A2 which was connected to each electrode (M1, M2)
Referring to FIG. 2, the lines A1 and A2 of the film side resistance membrane 30 are connected with an A/D converter 2. The A/D converters 2 convert the analogs electric potential of the point Pg of the glass side resistance membrane 31, into a digital signal corresponding to the Y coordinate. The CPU3 calculates the Y coordinate from the digital signal from the A/D converter 2. For example, a position of electrode M3 has a starting point of Y coordinate (Y=0) and electrode M4 has a terminal of Y coordinate (Y=1024). The intervals from electrode M3 to electrode M4 into a value of 1024 dots.
The basic operation of coordinate detection resistance membrane tablet with four electrodes is the same as mentioned above. But actually, it is needed to consider the resistance division 70 (R1) and 71 (R2) of the film side resistance membrane 30 when detecting an X coordinate, and it is needed to consider the resistance division 72 (R3) and 73 (R4) of the glass side resistance membrane 31 when detecting a Y coordinate. In addition, the value of each resistance division (70,71,72,72) changes according to the temperature change and a change of contact state of between each line (A1,A2,A3,A4) and the connector 44.
For example, a potential drop of 0.5 volts occurs by the contact resistance 70 of the line A1 connected to the electrode M1, and a potential drop of 0.5 volts occurred by the contact resistance 71 of the line A2 connected to the electrode M2. In this state, the value of electrical potential of the electrode M1 is 4.5 volts, and the value of electrical potential of the electrode M2 is 0.5 volts. Accordingly a difference of electrical potential between the electrode M1 and the electrode M2 becomes 4.0 volts. In this case, when electrical current potential Vf=4.5 volts was detected, the CPU3 needs to calculate the X coordinate as X=0. When an electrical current potential Vf=0.5 volts was detected, the CPU3 needs to calculate the X coordinate as X=1024. And by range of difference 4.0 volts of electrical potential with the electrode M1 and the electrode M2, the CPU3 needs to assign a range of X coordinates of 1024 dots.
The next expression (11 shows the difference in electrical potential of the electrodes M1 and M2. EQU Vfv=Rf/(Rf+R1+R2).times.V (11)
The next time, each contact resistance of lines A1 and A2 changes, as a result a potential drop of 0.7 volts occurs by the contact resistance 70 and a potential drop of 0.8 volts occurs by the contact resistance 71. By this state, a value of electrical potential of the electrode M1 is 4.3 volts, and a value of electrical potential of the electrode M2 is 0.8 volts. Accordingly a difference of electrical potential between the electrode M1 and the electrode M2 becomes 3.5 volts. In this case, when an electrical potential Vf=4.3 volts was detected, the CPU3 needs to calculate the X coordinate as X=0. When an electrical current potential Vf=0.7 volts was detected, the CPU3 needs to calculate the X coordinate as X=1024. Using the difference of 3.5 volts between the electrode M1 and the electrode M2, the CPU3 assigns a range of X coordinates of 1024 dots.
For the detection of Y coordinates, the CPU3 needs to consider the contact resistance of the electrode M3 and the electrode M4. The CPU3 needs to calculate an X coordinate with X=1024. Using the difference of electrical potential between electrodes M3 and M4, the CPU3 assigns a range of X coordinate of 1024 dots. Accordingly, before detecting an X coordinate and a Y coordinate, the CPU3 detects the value of electrical potential of each electrode as having given the voltage V.
However, in case of the coordinate input system of resistance membrane tablet of four electrodes, the value of electrical potential of each electrode (M1, M2, M3, M4) is taken out from each line (A1, A2, A3, A4) connected to each electrode and having the contact resistance (R1, R2, R3, R4). Accordingly, it can't detect the electrical potential of each electrode precisely because it can't detect the potential drop of each line with this hardware. The coordinate input system with a resistance membrane tablet of four electrodes must detect each potential drop of each line with software.
Next, a coordinate detection operation is explained for the coordinate inputting system with resistance membrane tablet of eight electrodes. FIG. 7 shows a structure of a resistance membrane tablet of the coordinate inputting system with a resistance membrane tablet of eight electrodes. The resistance membrane tablet with eight electrodes detects the potential drop of each line with hardware. Accordingly, it can detect the electrical potential of each electrode with hardware precisely.
FIG. 9 shows an equivalent circuit of the resistance membrane tablet with eight electrodes. The film side resistance membrane 30 includes, in addition to the electrodes M1 and M2 for the voltage V supply, electrodes N1 and N2 for electrical potential detection. The glass side resistance membrane 31 includes, in addition to the electrode M3 and M4 for the voltage V supply, electrodes N3 and N4 for electrical potential detection.
Next, a coordinate detection operation is explained for the coordinate inputting system using a resistance membrane tablet with eight electrodes. When it detects the X coordinate, the switches SW1 and SW2 are closed. In this condition, the circuit containing the resistance division 70 (R1) and 71 (R2) and the film side resistance membrane 30 (Rf) is closed. Then it supplies a voltage V to each electrode M1 and M2 at the ends of the film side resistance membrane 30 (Rf) through lines A1 and A4. When the stylus pen PE comes contacts the point Pf of the film side resistance membrane 30, the electrical potential of the point Pf is detected by the electrodes N3 and N4. Both electrical potentials detected by the electrodes N3 and N4 are equal. The electrical potential detected with electrode N3 or N4 is taken out from a line A6 or A7, and is output to the A/D converter 2.
When it detects the Y coordinate, the switches SW3 and SW4 are closed. In this condition, the circuit containing the resistance division 72 (R3) and 73 (R4), and the glass side resistance membrane 31 (Rg) is closed. Then it supplies a voltage V to each electrode M3 and M4 at the ends of the glass side resistance membrane 31 (Rg) through lines A5 and A8. When the stylus pen PE contacts the point Rf of the film side resistance membrane 30, the electrical potential of the point Pg is detected by the electrodes N1 and N2. Both electrical potentials detected by the electrodes N1 and N2 are equal. The electrical potential detected with the electrode N1 or N2 is taken out from a line A2 or A3, and is output to the A/D converter 2.
Next, an operation to detect an initial value of the electrical potential of each electrode (M1, M2, M3, M4) with a hardware is explained. When it detects, the electrical potentials of the electrodes N1 and the electrode N2, the switches SW1 and SW2 are closed, and it supplies a voltage V to each electrode M1 and M2 through lines A1 and A4. The electrical potential of the electrode N1 is detected on the line A2. The electrical potential on the electrode N2 is detected on the line A3. The electric current does not flow in the line A2 and the line A3. Accordingly, because the voltage drop from contact resistance 80 and contact resistance 81 does not occur, the electrical potential on each line A2 and A3 is almost the same as the electrical potential of each electrode N1 and N2. When it detects the electrical potential of the electrodes N3 and N4, the switches SW3 and SW4 are closed, and a voltage V appears at each electrode M3 and M4. The electrical potential of the electrode N3 is detected on the line A6. The electrical potential of the electrode N4 is detected on the line A7. The electric current does not flow in the lines A6 and A7. Accordingly, because the voltage drop from contact resistance 82 and contact resistance 83 does not occur, the electrical potential on each line A6 and A7 is almost the same as the electrical potential of each electrode N3 and N4.
In this way, the resistance membrane with eight electrodes detects each potential drop of each line with hardware. But the area of the electrode part of the tablet with eight electrodes is about 2 times the area of the electrode part of the tablet with four electrodes. The coordinate input system with four electrodes can not detect each potential drop of each line with hardware. Accordingly, when the voltage drop of a line changes because of change of contact resistance, it can not detect accurately the voltage drop with hardware. The CPU2 then can not calculate the value of the X and Y coordinates precisely.